I built my bus for full time use in all seasons.
With absolutely no insulation in ours, we could have a full sized (NOT a mini) wood stove going full tilt in winter and not even make a dent in the interior temperature. The metal surfaces all acted as conduits to the exterior. Ultimately I committed to full demolition/insulation.
If at all possible I recommend covering up all metal surfaces, removing the ceiling, the inner walls, all of the stock insulation, and as many original bus side windows as you are willing (RV windows can be added after). Proper demolition makes the longer term prospects much better- you will use less fuel/energy both heating and cooling.
Another thing to consider adding is sound insulation applied directly to the exposed walls after rust remediation and painting, before any other insulation/framing members.
Keep in mind that inadequate insulation can mean a miserable experience, frozen/damaged water lines, etc. There is virtually no such thing as too much insulation, if you add it you will enjoy your hard work all that more.